U.S. Pat. No. 4,261,581, assigned to Durametallic Corporation, discloses a mechanical seal construction employing a pair of opposed face rings which are disposed in relatively rotatable and sliding sealing engagement with one another. One face ring is fixedly related relative to a surrounding housing, and the other surrounds and is nonrotatably mounted relative to a rotating shaft. The rotating face ring is typically spaced radially outwardly from the shaft to define a substantial annular clearance space therebetween, and this face ring is coupled to one end of an axially elongate bellows which surrounds the shaft and at its other end is coupled to a collar which is fixedly and sealingly coupled to the shaft. This known mechanical seal construction is widely and successfully utilized in many environments, such as the chemical and petrochemical industries, requiring a sealing relationship for confining fluids, particularly corrosive and/or high-temperature fluids.
In the known and extensively utilized mechanical seal construction described above, the face rings are normally and preferably constructed of a carbide material, generally either silicon carbide or tungsten carbide, so as to both withstand the desired operating conditions and provide a desirable seal life. However, due to the extreme hardness of such carbide face rings, they have necessarily been maintained in substantial radially spaced relationship from the shaft so as to prevent contact therebetween due to the mechanical vibration which exists in the conventional mechanical seal environment, which vibration between the face rings and the shaft occurs in numerous forms and modes, including axial, radial and torsional vibrations.
Since such mechanical vibrations have long created problems with respect to wear and durability of mechanical seals, particularly those constructions involving metal bellows, seal designers have attempted to utilize various structural modifications in the seal construction so as to more closely control and confine the rotating seal parts relative to the shaft to prevent wear or failure due to such vibration. In one commonly and long utilized construction, the support ring for the rotating face ring is provided with several (normally three) tabs formed integrally thereon and projecting radially thereof so as to create a very close fit with the shaft, the clearance between these tabs and the shaft typically being a few thousands of an inch. These tabs and their close clearance with the shaft are intended to provide a much closer confinement, at least radially, of the rotating seal parts relative to the shaft, and hopefully minimize vibration. The Assignee's experience with this type of vibration dampening technique, however, through both field experience and laboratory testing, indicates that this vibration dampening construction creates more problems than it solves. For example, fretting damage between the shaft and the vibration dampening tabs occurs due to misalignment of the seal faces with respect to the shaft axis. Forced vibration of the support ring back and forth (that is, axially) also frequently results in fretting damage to the pump shaft. This damage and the adverse loads imposed on the support ring may be great enough to cause face separation, undesirably high leakage rates, or even hang up of the bellows. Close clearances of the vibration dampener tabs with the shaft also make the seal construction susceptible to hang up due to crystals or solids forming on the atmospheric side of the seal or thermal expansion of the shaft inside the vibration dampener tabs. Thus, this type of vibration dampening structure is, in many use situations, undesirable or unacceptable.
In an attempt to overcome the fretting and hang up problems caused by a vibration damper of the aforementioned type, mechanical seal constructions have also used a vibration damper constructed of a plastics or elastomeric material. Such vibration damper is generally comprised of a ring member which is axially rather thin, and which is fixedly clampingly held between the rotating face ring and its support ring. This elastomeric dampener ring projects radially inwardly so as to create a close fit with the shaft. While this elastomeric dampener ring obviously eliminates the fretting problem, nevertheless it has been observed to create other operational problems. For example, this elastomeric dampener has been observed to hang up on the shaft and thus effectively act as a seal, thereby interfering with proper operation of the mechanical seal construction. The elastomeric dampener also undergoes deterioration, particularly in view of the highly corrosive or high temperature fluids with which seals of this type are commonly used, and in addition the known dampeners have been of very short axial extent and accordingly have performed with only limited success.
In a very small number of use conditions, a mechanical seal construction in provided with carbon face rings rather than carbide face rings. Such carbon face rings are typically not used since they have a very short operational wear life. However, in situations where they are used, they have been constructed so as to have an inside diameter which provides only a minimal clearance relative to the shaft to minimize the vibration problem. While such a carbon face ring has been observed to operate in a satisfactory manner with respect to minimizing vibration problems, nevertheless such is not a fair evaluation of the overall seal construction performance, including an evaluation of the vibration problem, since such carbon face ring itself exhibits a very poor life cycle, such that the vibration problem itself may no longer be of primary concern.
Accordingly, it is an object of this invention to provide an improved bellows-type mechanical seal construction employing carbide face rings, which seal construction overcomes many of the above-mentioned disadvantages and operational problems.
In the improved bellows-type mechanical seal construction of this invention, the rotating carbide face ring has a vibration damper mounted thereon, the latter preferably being constructed as a carbon ring which is fixedly mounted within the carbide face ring, preferably by means of a press fit. The carbon vibration dampener ring is of substantial axial extent, and provides a close clearance with the shaft to effectively dampen vibrations of the rotating seal parts while at the same time avoiding damage to or hang up on the shaft.
In the improved seal construction, as briefly summarized above, the carbon dampener ring preferably has an axially length which is somewhat shorter then the axial length of the carbide face ring, the latter being supportingly positioned so that it has a nose part which projects axially beyond the carbon dampener ring, with this nose part defining thereon a seal face which slidingly contacts an opposed seal face on the stationary face ring. This arrangement thus prevents build up of coke and debris in or directly adjacent the plane of the seal face.
With the improved seal construction of this invention, a bellows-type seal is able to retain use of carbide face rings so as to provide optimum life, and at the same time the face ring can be desirably closely and concentrically supported relative to the shaft by means of the intermediate carbon dampener ring so as to effectively dampen vibrations and minimize problems caused by such vibration, and at the same time the carbon dampener ring effectively provides a long and concentric area for supporting engagement with the shaft, which engagement area is effectively self lubricating and does not create any fretting of the shaft or hang up of the seal parts. At the same time, the carbon ring is able to maintain the desirable and necessary minimal clearance between the carbon dampener ring and the shaft so as to not create a total seal at this point, and still permit the necessary floating movement (both radial and angular float) of the face ring as required in order to achieve optimum seal performance.
Other structural features, objects and purposes of the invention will be apparent to persons familiar with seal constructions of this general type upon reading the following specification and inspecting the accompanying drawings.